In cellular networks, certain metrics may be used for control or monitoring purposes. For example, it is known to perform association of a user equipment (UE) to a particular cell using a metric which generally reflects the channel quality of between the UE and a base station serving the cell. For example, in the LTE (Long Term Evolution) radio technology specified by 3GPP (3rd Generation Partnership Project), cell association is typically done with metrics referred to as Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ). The cell association using these metrics may involve selecting a cell to which the UE attaches, e.g., in idle mode, or performing a handover (HO) between two cells while maintaining an active data connection.
The RSRP and RSRQ procedures for their measurement are for example defined in 3GPP TS 36.214 V1.1.0 (2013 December). The cell association processes in idle mode are for example specified in 3GPP TS 36.133 V12.1.0 (2013 September) and 3GPP TS 36.304 V11.5.0 (2013 September). Procedures for active mode HOs are specified in 3GPP TS 36.331 V11.5.0 (2013 September).
The RSRP is defined to be measured by the UE as the linear average over the power contributions of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.
The RSRQ is defined as the ratio N×RSRP/(E-UTRA carrier RSSI), where the E-UTRA carrier RSSI (Received Signal Strength Indicator) comprises the linear average of the total received power observed only in OFDM (Orthogonal Frequency Division Multiplexing) symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over N resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise, etc.
A typical cell selection process may operate to select the cell for association which has the highest RSRP or RSRQ.
However, performing cell association by selecting the cell with the highest RSRP or RSRQ may in some scenarios provide undesirable results. For example, the cell with the highest RSRP or cells adjacent thereto may be highly loaded, which may adversely affect the performance when serving the UE in this cell, because the high load tends to cause more interference and unsatisfactory user experience. The RSRQ is sensitive to the traffic load in the cell and neighboring cells. Accordingly, since the instantaneous traffic load can fluctuate very quickly, selecting the cell with the highest RSRQ may result in unstable cell selection decisions, e.g., leading to ping-pong HOs.
Further, selecting the cell with the highest RSRP or RSRQ may be problematic in scenarios where the noise floor is different on different frequency channels and/or in different frequency bands, or is different between cells on the same frequency. This could be the case when the noise includes thermal noise and external interference from other radio systems operating on the same or on adjacent frequencies. For example some users of radio spectrum, e.g., TV broadcasters, do not use their allocated frequency bandwidth completely at all times and at all locations and leave certain regions of their allocated bandwidth unused at given locations. These unused spectra are referred to as “white spaces”. Such white space spectrum can under certain circumstances be used by a cell of the cellular network. In such cases the interference situation on a channel in the white space spectrum may differ significantly from a channel in the spectrum which is dedicated to the cellular network, e.g., due to interference generated by the original user of the white space spectrum.
Further, selecting the cell with the highest RSRP or RSRQ may be problematic in heterogeneous network scenarios where a macro cell layer is deployed on one frequency channel and a pico cell layer is deployed on another frequency channel and these two frequency channels are located in different frequency bands. For example, the frequency channel of the pico cell layer may be located in a white space spectrum. Also in this case, the cells of the pico layer may experience an interference situation which differs significantly from that of the macro layer.
Still further, due to UE receiver imperfections RSRP measurements are likely to be biased by received interference and noise power. That is to say, the interference and noise may have the effect that the measured value of the RSRP does not accurately represent the strength of the reference signals, but is too high. This may have the effect that a cell having a stronger impact from interference and noise is preferred over a cell with otherwise equal or even higher RSRP. This is of course not desirable.
Also in the case of other control purposes, usage of the RSRP or RSRQ may be problematic if the interference situation varies significantly between different cells or spectra utilized by the cellular network.
Accordingly, there is a need for techniques which allow for improved control of cellular network operation.